In persons with aortic regurgitation, the aortic pressure pulse is increased significantly, there is no incisura in the aortic pulse contour, and the pressure could be 160/35 mm Hg., In normal subjects the aortic pressure pulse could be 40 mm Hg, the sharp incisura is present in the aortic pulse contour and the pressure could be 120/80 mm Hg, In persons with aortic valve stenosis, the aortic pressure pulse is decreased significantly, and the pressure could be 115/85 mm Hg , In persons with patent ductus arteriosus, the aortic pressure pulse is increased significantly, the sharp incisura is present in the aortic pulse contour, and the pressure could be 162/38 mm Hg., In persons with arteriosclerosis, the aortic pressure pulse is increased significantly, the sharp incisura is present in the aortic pulse contour, and the pressure could be 170/95 mm Hg.

It is rare that any single functional cell of the body is more than 20 to 30 micrometers away from a capillary., The walls of the capillaries are thin and constructed of single-layer, highly permeable endothelial cells., The most purposeful function of the microcirculation is transport of nutrients to the tissues and removal of cell excreta., The small arterioles control blood flow to each tissue, The local conditions in the tissues control the diameters of the arterioles.

Each tissue, in most instances, controls its own blood flow in relation to its individual needs. , The terminal arterioles do not have smooth muscle fibers. , The microcirculation of each organ is organized to serve that organ’s specific needs, The metarterioles do not have a continuous muscular coat, but smooth muscle fibers encircle the vessel at intermittent points , The arterioles are highly muscular, and their diameters can change manyfold.

Arterioles have strong muscular walls that can, by relaxing, dilate the vessels severalfold. , The venules collect blood from the capillaries and gradually coalesce into progressively larger veins. , Arterioles have strong muscular walls that can close the arterioles completely. , The arterioles are the last small branches of the arterial system., The venules are larger than the arterioles and have a much weaker muscular coat.

The intercellular clefts are formed from oligomers of caveolins associated with of cholesterol and sphingolipids., The intercellular clefts are located only at the edges of the endothelial cells., The intercellular clefts, are interrupted periodically by short ridges of protein attachments that hold the endothelial cells together. , Water molecules, as well as most water-soluble ions and small solutes, diffuse with ease through the intercellular clefts. , The intercellular cleft is the thin-slit, curving channel that lies between adjacent endothelial cells.

The extracellular fluid is mixed between the blood and the tissue fluids by diffusion through the capillary walls. , Water and dissolved molecules are continually moving through the capillary pores., The internal diameter of the capillary is barely large enough for red blood cells and other blood cells to squeeze through. , The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid. , Capillary walls are thin and have numerous minute capillary pores permeable to water.

Caveolae form from oligomers of proteins called caveolins that are associated with molecules of cholesterol and sphingolipids., Caveolae on the cell surface can take up plasma with plasma proteins and then move slowly through the endothelial cell. , Caveolae are believed to play a role in endocytosis and transcytosis of macromolecules across the interior of the endothelial cells. , Caveolae are minute plasmalemmal vesicles present in the endothelial cells. , Some of these plasmalemmal vesicles may coalesce to form vesicular channels all the way through the endothelial cell.

In the brain, the junctions between the capillary endothelial cells are mainly “tight” junctions., In all capillaries, lipid-soluble substances diffuse directly through the cell membranes of the capillary without having to go through the pores., In the liver, clefts between the capillary endothelial cells are wide open so that almost all dissolved substances of the plasma can pass from the blood into the liver tissues., The pores of the gastrointestinal capillary membranes are midway in size between those of the muscles and those of the liver., In the glomerular capillaries of the kidney, numerous fenestrae penetrate all the way through the middle of the endothelial cells.

The most important factor affecting the degree of opening and closing of the metarterioles and precapillary sphincters is the concentration of oxygen in the tissues. , Because of vasomotion, blood usually does not flow continuously through the capillaries., In each tissue capillary bed, the average functions of capillaries represent the functions of literally billions of individual capillaries, each operating intermittently in response to local conditions in the tissues., When the rate of oxygen usage by the tissue is great, the intermittent periods of capillary blood flow occur more often., The vasomotion, means intermittent contraction of the metarterioles and precapillary sphincters (and sometimes even the very small arterioles).

Many substances needed by the tissues are soluble in water but cannot pass through the lipid membranes of the endothelial cells. , If a substance is lipid soluble, it can diffuse directly through the cell membranes of the capillary without having to go through the pores., Diffusion results from thermal motion of the water molecules and dissolved substances in the fluid., Oxygen and carbon dioxide rates of transport through the capillary membrane are many times faster than the rates for lipid-insoluble substances. , The most important means by which substances are transferred between the plasma and the interstitial fluid is diffusion.

The permeability for glucose molecules is 0.6 times that for water molecules., The permeability for albumin molecules is very slight only 1/1000 that for water molecules. , The relative permeabilities of the capillary pores in skeletal muscle is highest for water and lowest for albumin., The diameters of plasma protein molecules, are slightly greater than the width of the pores. , The water molecule is the smallest molecule that normally passes through the capillary pores.

The rate at which water molecules diffuse through the capillary membrane is greater than the rate at which plasma itself flows linearly along the capillary. , Great degrees of capillary permeability are required for the kidneys to allow filtration of large quantities of fluid for the formation of urine., The permeability of the renal glomerular membrane for water and electrolytes is about 500 times the permeability of the muscle capillaries. , Great degrees of capillary permeability are required for the liver to transfer nutrients between the blood and hepatocytes. , The membranes of the liver capillary sinusoids are so permeable that even plasma proteins pass through these walls.

Excess carbon dioxide moves to the blood and is carried away from the tissues. , The concentration of carbon dioxide is greater in the tissues than in the arterial blood., The greater the concentration gradient of a substance, the greater the net movement of that substance through the membrane., The “net” rate of diffusion of a substance through any membrane is proportional to the concentration difference of the substance between the two sides of the membrane., Large quantities of oxygen normally move from the blood toward the tissues.

The structure of the interstitium contains collagen fiber bundles and proteoglycan filaments., Proteoglycan filaments are everywhere in the spaces between the collagen fiber bundles. , The fluid in interstitium is called the interstitial fluid., The collagen fiber bundles are extremely strong and therefore provide most of the tensional strength of the tissues., The collagen fiber bundles extend long distances in the interstitium.

The interstitial fluid is entrapped mainly in the minute spaces among the proteoglycan filaments., This combination of proteoglycan filaments and fluid entrapped within them has the characteristics of a gel (tissue gel). , The fluid in the interstitium is derived by filtration and diffusion from the capillaries. , The fluid in the interstitium contains almost the same constituents as plasma except for much lower concentrations of proteins. , Because of the large number of proteoglycan filaments, it is difficult for fluid to flow easily through the tissue gel.

Fluid filtration across capillaries is determined by hydrostatic and colloid osmotic pressures and the capillary filtration coefficient. , The lymphatic system, which returns to the circulation the small amounts of excess protein and fluid that leak from the blood into the interstitial spaces. , The osmotic pressure exerted by the plasma proteins normally prevents significant loss of fluid volume from the blood into the interstitial spaces., The hydrostatic pressure in the capillaries tends to force fluid and its dissolved substances through the capillary pores into the interstitial spaces. , The colloid osmotic pressure tends to cause fluid movement by osmosis from the interstitial spaces into the blood

The capillary plasma colloid osmotic pressure (Πp) tends to cause osmosis of fluid inward through the capillary membrane. , The interstitial fluid pressure (Pif) tends to force fluid inward through the capillary membrane when is positive., The interstitial fluid colloid osmotic pressure (Πif) tends to cause osmosis of fluid outward through the capillary membrane., The capillary pressure (Pc) tends to force fluid outward through the capillary membrane. , The interstitial fluid pressure (Pif) tends to force fluid outward through the capillary membrane when is negative.

The number and size of the pores in each capillary are usually expressed as the capillary filtration coefficient. , Capillary filtration coefficient is therefore a measure of the capacity of the capillary membranes to filter water for a given net filtration pressure , The filtration rate of the capillary fluid is determined by multiplying the net filtration pressure by the capillary filtration coefficient., The rate of fluid filtration in a tissue is partly dependent on the number and size of pores in each capillary., The net filtration pressure is positive in normal conditions, resulting in filtration of fluid across the capillaries into the interstitial space in most organs.

The pressure in intrapleural space is averaging about −8 mm Hg., The brain interstitial fluid pressure averages about +4 to +6 mm Hg. , The renal interstitial fluid pressures averaged about +6 mm Hg. , Interstitial fluid pressure in loose subcutaneous tissue is averaging about −3 mm Hg. , The cerebrospinal fluid pressure surrounding the brain of an animal lying on its side averages about +10 mm Hg

Plasma colloid osmotic pressure = 28 mm Hg , Net outward force (at arterial end) = 13 mm Hg , Negative interstitial free fluid pressure = 3 mm Hg , Capillary pressure (arterial end of capillary) = 30 mm Hg , Interstitial fluid colloid osmotic pressure = 8 mm Hg

Interstitial fluid colloid osmotic pressure = 8 mm Hg , Net reabsorption pressure at the venous ends of the capillaries = 7 mm Hg , Negative interstitial free fluid pressure = 3 mm Hg , Plasma colloid osmotic pressure = 28 mm Hg , Capillary pressure (venous end of capillary) = 10 mm Hg

Net filtration pressure = 0.3 mm Hg , Negative interstitial free fluid pressure = 3 mm Hg , Plasma colloid osmotic pressure = 28 mm Hg , Mean capillary pressure = 17.3 mm Hg , Interstitial fluid colloid osmotic pressure = 8 mm Hg

The lymphatic system represents an accessory route through which fluid can flow from the interstitial spaces into the blood. , The lymphatic system is a “scavenger” system that removes excess fluid, excess protein molecules, debris, and other matter from the tissue spaces., The lymphatic system is one of the major routes for absorption of nutrients from the gastrointestinal tract., The lymphatics can carry proteins and large particulate matter away from the tissue spaces., The return of proteins to the blood from the interstitial spaces is an essential function of the lymphatic system.

Some body tissues, which are surrounded by tight membranes, have positive interstitial fluid pressures. , In most natural cavities of the body where there is free fluid in dynamic equilibrium with the surrounding interstitial fluids, the pressures that have been measured have been negative. , Interstitial fluid pressure in loose subcutaneous tissue is usually subatmospheric., Pumping by the lymphatic system is the basic cause of the negative interstitial fluid pressure., In the kidneys, the capsular pressure surrounding the kidney averages about +13 mm Hg.

About 20 milliliters of lymph flows into the circulation each hour through other lymphatic channels than the thoracic duct. , About 100 milliliters per hour of lymph flows through the thoracic duct of a resting human. , Normal lymph flow is very little at interstitial fluid pressures more negative than the normal value of −6 mm Hg., Lymph flow reaches a maximum when the interstitial pressure rises slightly above atmospheric pressure (0 mm Hg). , The total estimated lymph flow of is about 120 ml/hr or 2 to 3 liters per day.

Decreased plasma colloid osmotic pressure , Elevated capillary hydrostatic pressure, Increased interstitial fluid colloid osmotic pressure , Any factor that increases interstitial fluid pressure , Increased permeability of the capillaries

Each segment of the lymph vessel between successive valves functions as a separate automatic pump, In a very large lymph vessel such as the thoracic duct, this lymphatic pump can generate pressures as great as 50 to 100 mm Hg., When a collecting lymphatic vessel becomes stretched with fluid, the smooth muscle in the wall of the vessel automatically contracts. , Valves exist in all lymph channels. , Even slight filling of a segment causes it to contract, and the fluid is pumped through the next valve into the next lymphatic segment.

Intrinsic intermittent contraction of the lymph vessel walls , Contraction of surrounding skeletal muscles. , Movement of the parts of the body. , Pulsations of arteries adjacent to the lymphatics. , Compression of the tissues by objects outside the body.

The excess fluid flows from the terminal lymphatic capillary causes the tissue to swell. , The lymphatic capillary endothelial cells contain a few contractile actomyosin filaments, When the tissue is compressed, the pressure inside the capillary increases and pushes the lymph forward into the collecting lymphatic, The terminal lymphatic capillary is capable of pumping lymph. , Each time excess fluid causes the tissue to swell, fluid flows into the terminal lymphatic capillary through the junctions between the endothelial cells

Only minute amounts, if any, of the leaked proteins return to the circulation by way of the venous ends of the blood capillaries, The lymphatic system functions as an “overflow mechanism” to return excess proteins and excess fluid volume from the tissue spaces to the circulation. , The lymphatic system plays a central role in controlling the interstitial fluid pressure., When the tissues lose their negative pressure, fluid accumulates in the spaces and the condition known as edema occurs, The tissues are held together by the negative interstitial fluid pressure, which is a partial vacuum

If the capillary pressure falls very low, net filtration of fluid into the capillaries will occur., One tenth of the filtered fluid enters the lymphatic capillaries and returns to the blood through the lymphatic system. , The rate of lymph flow is determined by the product of interstitial fluid pressure times the activity of the lymphatic pump. , The lymphatic system returns to the circulation the small amounts of excess protein and fluid that leak from the blood into the interstitial spaces., Osmotic pressure caused by the plasma proteins normally prevents significant loss of fluid volume from the blood into the interstitial spaces.

Transport of various hormones and other substances to the different tissues, Delivery of other nutrients such as glucose, amino acids, and fatty acids, Delivery of oxygen to the tissues., Keep of proper concentrations of ions in the blood., Maintenance of proper concentrations of ions in the tissues.

Long-term control of blood flow is a result of an increase or decrease in the physical sizes and numbers of blood vessels supplying the tissues. , Acute control is achieved by rapid changes occurring within seconds to minutes to ensure very rapid maintenance of adequate local tissue blood flow. , Long-term control of blood flow means slow, controlled changes in flow over a period of days, weeks, or even months. , Long-term blood flow control provides a good control of blood flow in proportion to tissue needs., Acute control is achieved by rapid changes in local vasodilation or vasoconstriction of the arterioles, metarterioles, and precapillary sphincters.

In cyanide poisoning , In pneumonia , All situations associated with reduced oxygen availability , In carbon monoxide poisoning , At the top of a high mountain

Carbon dioxide , Potassium ions , Histamine, Hydrogen ions , Adenosine and adenosine phosphate compounds

Whenever the heart becomes more active than normal, adenosine leaks out of the heart muscle cells to cause coronary vasodilation, Decreased availability of oxygen can cause both adenosine and lactic acid to be released into the spaces between the tissue cells. , An increase in the concentration of vasodilator metabolites causes vasodilation of the arterioles, thus increasing the tissue blood flow. , Vasodilator substances increase in the tissues when cell metabolism is suddenly increased., Vasodilator substances may be released from the tissue in response to oxygen deficiency.

Reactive hyperemia: lack of blood flow sets into motion all of the factors that cause vasodilation., “Active hyperemia”: when any tissue becomes highly active, the rate of blood flow through the tissue increases. , After short periods of vascular occlusion, the extra blood flow during the reactive hyperemia phase lasts long enough to repay almost exactly the tissue oxygen deficit., Active hyperemia in skeletal muscle can increase local muscle blood flow as much as 20-fold during intense exercise. , When tissue metabolic rate increases, such as the brain during increased mental activity, “active hyperemia” occurs.

Autoregulation could be sustained by myogenic contraction initiated by stretchinduced vascular depolarization., Autoregulation means the return of flow toward normal, after initial increase caused by rapid increase in arterial pressure., Within less than a minute after the rapid increase in blood flow caused by suddenly increase of arterial pressure, the blood flow returns almost to the normal level., In any tissue of the body, a rapid increase in arterial pressure causes an immediate rise in blood flow., The metabolic theory of autoregulation sustains that too much oxygen and less vasodilators cause the blood vessels to constrict and return flow to nearly normal.

Even with severe vasoconstriction, skin blood flow is usually great enough to meet the basic metabolic demands of the skin., In the skin, blood flow control is closely linked to regulation of body temperature. , In the brain, an excess carbon dioxide or hydrogen ions from the brain tissues dilates the cerebral vessels. , In the brain, the level of excitability is highly dependent on exact control of both carbon dioxide concentration and hydrogen ion concentration., In the kidneys, tubuloglomerular feedback initiated by signals from the macula densa modulate glomerular filtration rate.

Endothelial derived nitric oxide synthase enzymes synthesize NO from arginine and oxygen and by reduction of inorganic nitrate., Faster degradation of cGMP sustains the actions of NO to cause vasodilation., The released NO increases the diameters of the larger upstream blood vessels whenever microvascular blood flow increases downstream., The most important of the endothelial derived relaxing factors is nitric oxide., When endothelial cells are damaged by chronic hypertension or atherosclerosis, impaired NO synthesis may contribute to excessive vasoconstriction.

Endothelin is a powerful vasoconstrictor released from damaged endothelium., The usual stimulus for endothelin release is damage to the endothelium. , Endothelin is present in the endothelial cells of all or most blood vessels but greatly increases when the vessels are injured. , Crushing the tissues or injecting a traumatizing chemical into the blood vessel stimulate release of endothelin. , Increased endothelin release is also believed to contribute to vasoconstriction when the endothelium is damaged by hypertension.

Lack of glucose in the perfusing blood can cause local tissue vasodilation , Vasodilation occurs in deficiencies of the vitamin B substances., Deficiency of vitamins necessary for oxygen induced phosphorylation might lead to diminished smooth muscle contractile ability and therefore local vasodilation as well. , When the excess oxygen is gone and the oxygen concentration falls low enough, the precapillary sphincters would open., The strength of contraction of the precapillary sphincters would increase with an increase in oxygen concentration.

The acute mechanisms for local blood flow regulation are fast but incomplete. , The long-term regulation gives far more complete control of blood flow as compare with acute control. , Once the long-term regulation has had time to occur, long-term changes in arterial pressure have little effect on the rate of local blood flow. , If a tissue becomes chronically overactive, the arterioles and capillary vessels usually increase both in number and size within a few weeks to match the needs of the tissue. , Long-term regulation of blood flow is especially important when the metabolic demands of a tissue change.

Actual physical reconstruction of the tissue vasculature occurs to meet the needs of the tissues., In the neonate, the vascularity will adjust to match almost exactly the needs of the tissue for blood flow., A key mechanism for long-term local blood flow regulation is to change the amount of vascularity of the tissues. , Angiogenesis occurs rapidly in cancerous tissue. , If the metabolism in a tissue is increased for a prolonged period, vascularity increases, a process generally called angiogenesis.

The vascularity is increased in tissues of animals that live at high altitudes, where the atmospheric oxygen is low., Oxygen is important for both acute control and long-term control of local blood flow. , Often so much overgrowth occurs in premature babies, that the retinal vessels grow out from the retina into the eye’s vitreous humor. , In premature human babies who are put into oxygen tents, the excess oxygen causes almost immediate overgrowth of the retinal vessels., When the infant is taken out of the oxygen tent, explosive overgrowth of new vessels then occurs to make up for the sudden decrease in available oxygen

Endostatin is an antiangiogenic peptide, Platelet derived growth factor increases growth of new blood vessels., Vascular endothelial growth factor increases growth of new blood vessels., Angiogenin increases growth of new blood vessels, Fibroblast growth factor increases growth of new blood vessels

Long-term increased of the tissue metabolism stimulates angiogenesis. , After extra vascularity does develop, the extra blood vessels opens when a lack of oxygen call forth the required extra flow., The need for increased blood flow produced by short term heavy exercise can cause enough angiogenic factors to increase muscle vascularity. , Vascularity is increased in tissues of animals that live at high altitudes, where the atmospheric oxygen is low. , Vascularity is determined mainly by the maximum level of blood flow need.

By the age of 40 years most people have experienced a heart attack, because the lack of collateral vessels. , An important example of the development of collateral blood vessels occurs after thrombosis of one of the coronary arteries., Development of collateral vessels involves rapid metabolic dilation followed chronically by growth and enlargement of new vessels over a period of weeks and months., In most tissues of the body, when an artery is blocked, a new vascular channel develops around the blockage and allows at least partial resupply of blood to the affected tissue. , When collateral blood vessels are unable to develop quickly enough to maintain blood flow because of the rapidity or severity of the coronary insufficiency, serious heart attacks occur.

The inward eutrophic remodeling increases the size of vascular smooth muscle cells. , Vascular growth and remodeling are critical components of the adaptive tissular response to long-term changes in blood pressure or blood flow., If the blood vessel is exposed to long-term increases in blood pressure and blood flow, there is usually outward hypertrophic remodeling., During inward eutrophic remodeling, there is an increase in the cross-sectional area of the vascular wall. , If blood vessels are exposed to chronic increases in blood flow, there is typically outward remodeling.

Norepinephrine is an especially powerful vasoconstrictor hormone. , Angiotensin II is powerful vasoconstrictor substance, Epinephrine dilates the coronary arteries during increased heart activity. , Vasopressin is one of the body’s most potent vascular constrictor substances. , Vasopressin helps to control body fluid volume.

Kinins regulating blood flow and capillary leakage of fluids in inflamed tissues, Bradykinin causes powerful arteriolar dilation, Bradykinin increases capillary permeability., The local vasodilatory and edema producing effects of histamine are especially prominent during allergic reactions, Kinins cause powerful vasodilation.

An increase in magnesium ion concentration causes powerful vasodilation. , Acetate and citrate, both of which cause mild degrees of vasodilation. , An increase in hydrogen ion concentration causes dilation of the arterioles. , An increase in potassium ion concentration, within the physiological range, causes vasodilation., An increase in calcium ion concentration causes vasoconstriction

Magnesium ions inhibit smooth muscle contraction, Increase in carbon dioxide concentration causes marked vasodilation in the brain., A slight decrease in hydrogen ion concentration causes arteriolar constriction, The general effect of calcium is to stimulate smooth muscle contraction. , Carbon dioxide in the blood has an indirect effect, causing widespread sympathetic - mediated vasodilatation throughout the body.

Blood flow is generally regulated according to the specific needs of the tissues as long as the arterial pressure is adequate to perfuse the tissues. , Each tissue can autoregulate its own blood flow according to the metabolic needs and other functions of the tissue, Most vasodilators cause only short-term changes in tissue blood flow and cardiac output if they do not alter tissue metabolism. , Tissue blood flow and cardiac output are not substantially altered, except for a day or two, by infusion of large amounts of powerful vasoconstrictors. , Angiotensin II may cause transient decreases in tissue blood flow and cardiac output but has little long-term effect if it does not alter metabolic rate of the tissues.

Nitric Oxide is a vasodilator released from healthy endothelial cells, Vasopressin is even more powerful than angiotensin II as a vasoconstrictor. , Norepinephrine is a powerful vasoconstrictor. , Adenosine is an important local vasodilator for controlling local blood flow , Angiotensin II is a powerful vasoconstrictor substance.

Histamine has a powerful vasodilator effect on the arterioles., Reduced oxygen availability increases tissue blood flow. , Endothelin is a powerful vasoconstrictor released from damaged endothelium. , Decrease in pH causes dilation of the arterioles, Bradykinin causes both powerful arteriolar dilation and increased capillary permeability.

Vascular endothelial growth factor is an angiogenic factor., Angiostatin is an angiogenic factor, Platelet derived growth factor is an angiogenic factor., Fibroblast growth factor is an angiogenic factor., Endostatin is an antiangiogenic factor.

Vasodilator substances may be released from the tissue in response to oxygen deficiency. , An increase of the carbon dioxide and/or hydrogen ions concentrations dilates the cerebral vessels. , The peripheral vascular blood flow increases in vitamin B deficiency., Adenosine and phosphate compounds are important local vasodilators for controlling local blood flow., When the availability of oxygen decreases the blood flow through the tissues increases.

Kinins cause powerful vasodilation when formed in the blood and tissue fluids., The effect of angiotensin II is to powerfully constrict the small arterioles. , An increase in calcium ion concentration causes vasoconstriction , In the kidneys, blood flow control is vested to a great extent in a mechanism called tubuloglomerular feedback. , When endothelial cells are damaged by chronic hypertension or atherosclerosis, impaired NO synthesis may contribute to excessive vasoconstriction

Angiotensin II → small arterioles constriction., ↑ K+ → vasodilation, Vascular endothelial growth factor → angiogenesis, Bradykinin → arteriolar dilation, Angiostatin → ↑ angiogenesis

↓ Mg++ → vasodilation. , cyanide poisoning → ↑ local blood flow., ↑ CO2 → ↑ local blood flow. , Angiotensin II → ↑ total peripheral resistance, Long-term ↑ in vascular wall tension → ↑ vascular wall thickness

Adjusts blood flow in the tissues., Coordinates increasing or decreasing pumping activity by the heart, Provides rapid control of systemic arterial pressure, Is realized almost entirely through the autonomic nervous system, Adjusts redistributing blood flow to different areas of the body

The most important part of the autonomic nervous system for regulating the circulation is the sympathetic nervous system, The substance secreted at the endings of the vasoconstrictor nerves is almost entirely norepinephrine, The hypothalamus can exert powerful excitatory or inhibitory effects on the vasomotor center, Sympathetic vasomotor nerve fibers pass immediately into a sympathetic chain, The parasympathetic nervous system contributes importantly to regulation of heart function

The innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow through the tissues., Sympathetic stimulation decreases the veins volume pushing blood towards the heart. , Sympathetic stimulation increases cardiac output. , This sympathetic vasoconstrictor effect is especially powerful in the kidneys, intestines, spleen, and skin., Sympathetic stimulation increases the heart rate and strength of pumping

The parasympathetic nervous system plays only a minor role in regulating vascular function in most tissues, Parasympathetic inhibition combined with sympathetic stimulation increases the pumping effectiveness of the heart. , The parasympathetic nervous system decreases heart rate by way of parasympathetic nerve fibers of the vagus nerves., Heart effects of parasympathetic system are mediated by acetylcholine., The acetylcholine released by parasympathetic stimulation has a direct effect to dilate the coronary arteries.

The vasomotor center transmits sympathetic impulses through the spinal cord and peripheral sympathetic nerves to virtually all arteries, arterioles, and veins of the body., The vasomotor center is in the reticular substance of the medulla and pons., Neurons from the vasoconstrictor area excite preganglionic vasoconstrictor neurons of the sympathetic nervous system. , The neurons from the vasodilator area inhibit the vasoconstrictor area and cause vasodilation. , The vasomotor center transmits parasympathetic impulses through the vagus nerves to the heart.

The sensory area neurons send output signals to control activities of both the vasoconstrictor and vasodilator areas of the vasomotor center. , The vasomotor center has a vasodilator area in the medulla. , The vasomotor center transmits parasympathetic impulses through the vagus nerves to the heart., The sensory area of the vasomotor center is in the nucleus tractus solitarius., The sensory area neurons receive signals from the circulatory system through the vagus and glossopharyngeal nerves.

Could be blocked during total spinal anesthesia, Maintain a partial state of contraction in the blood vessels, called vasomotor tone., Results from infusion of a small amount of the hormone norepinephrine., Causes a continuous partial constriction of the blood vessels., Caused by the continual firing of the sympathetic vasoconstrictor nerve fibers.

The lateral portions of the vasomotor center increase heart rate and contractility., The vasomotor center can increase heart rate and strength of contraction when vasoconstriction occurs. , At the same time that the vasomotor center regulates the amount of vascular constriction, it also controls heart activity., The medial portion of the vasomotor center sends signals to the adjacent dorsal motor nuclei of the vagus nerves to decrease heart activity., The vasomotor center regulates the amount of vascular constriction.

Parasympathetic stimulation causes a marked decrease in heart rate and a slight decrease in heart muscle contractility. , At the level of the heart, sympathetic and parasympathetic stimulation have opposite effects., All the vessels except the capillaries are innervated by sympathetic nerve fibers. , Parasympathetic stimulation is transmitted to the heart via the vagus nerves., Sympathetic stimulation increases heart rate and contractility.

Sympathetic impulses cause the adrenal medullae to secrete both epinephrine and norepinephrine into the circulating blood., Acetylcholine is the parasympathetic neurotransmitter., Norepinephrine is the sympathetic vasoconstrictor neurotransmitter. , Norepinephrine acts directly on the alpha adrenergic receptors of the vascular smooth muscle to cause vasoconstriction , Acetylcholine, slows the heart and has a slight depressive effect on heart contractility

The sympathetic nerves to skeletal muscles carry sympathetic vasodilator fibers as well as constrictor fibers., The vasodilator response in skeletal muscles may be mediated by nitric oxide released from the vascular endothelium in response to stimulation by acetylcholine, The cerebral cortex, the anterior hypothalamus, the vagal centers of the medulla, and the spinal cord are involved in mechanism of vasovagal syncope. , The sympathetic vasodilator effect is believed to be caused by epinephrine exciting specific beta-adrenergic receptors in the muscle vasculature., The vasovagal syncope appears when the muscle vasodilator system becomes activated and the vagal cardioinhibitory center decreases the heart rate.

Is one of the most important functions of nervous control of the circulation., The heart is directly stimulated by the autonomic nervous system enhancing cardiac pumping, The veins especially are strongly constricted., Most arterioles of the systemic circulation are constricted., The vasoconstrictor and cardioaccelerator functions of the sympathetic nervous system are stimulated at the same time with inhibition of parasympathetic system

The alarm reaction provides an excess of adrenaline to contract muscles of the body. , The increase in arterial pressure during exercise results mainly from effects of the nervous system., The alarm reaction means rapidly increase of the arterial pressure by as much as 75 to 100 mm Hg during extreme fright. , Stimulation of the vasoconstrictor and cardioacceleratory areas of the vasomotor center is happening at the same time that the motor areas of the brain become activated. , The increase in muscle blood flow during intense exercise is partly determined by the increase in arterial pressure caused by sympathetic stimulation.

The baroreceptor reflex is initiated by stretch receptors. , In the normal operating range of arterial pressure even a slight change in pressure causes a strong change in the baroreflex signal to readjust arterial pressure back toward normal. , The baroreceptor reflex is the best known of the nervous mechanisms for arterial pressure control., “Feedback” signals of the baroreceptor reflex are sent through the autonomic nervous system to reduce arterial pressure downward toward the normal level. , A risein arterial pressure stretches the baroreceptors and causes them to transmit signals into the CNS

In the normal operating range of arterial pressure even a slight change in pressure causes a strong change in the baroreflex signal to readjust arterial pressure back toward normal. , Baroreceptors are extremely abundant in the carotid sinus and the wall of the aortic arch., A few baroreceptors are located in the wall of almost every large artery of the thoracic and neck regions., Signals from the “carotid baroreceptors” are transmitted through small Hering’s nerves to the glossopharyngeal nerves, and then to the nucleus tractus solitarius., Signals from the “aortic baroreceptors” in the arch of the aorta are transmitted through the vagus nerves to the nucleus tractus solitarius.

The decrease in the inhibitory effect of the nucleus tractus solitarius on the activity of the vasomotor center is followed by the increase in arterial pressure., The decrease in pressure in the carotid sinus is followed by a decrease of baroreceptors firing rate and a smaller inhibitory effect on the vasomotor center., In response to baroreceptor stimulation, the nucleus tractus solitarius inhibits the vasoconstrictor center of the medulla and excites the vagal parasympathetic center., The net effects of the nucleus tractus solitarius signals are vasodilation of the veins and arterioles, simultaneously with decreased heart rate and strength of heart contraction. , Activation of the baroreceptor reflex by high arterial pressure causes a reflex decrease in pressure due to the decrease of both peripheral resistance and cardiac output.

The effect of the abdominal compression reflex on the circulation is an increase in both cardiac output and arterial pressure, People whose skeletal muscles have been paralyzed are considerably more prone to hypotensive episodes than are people with normal skeletal muscles. , Abdominal muscle contraction compresses all the venous reservoirs of the abdomen, helping to translocate blood out of the abdominal vascular reservoirs toward the heart. , The abdominal compression reflex involves the contraction of the abdominal muscles, making increased amounts of blood available to be pumped by the heart. , When a baroreceptor or chemoreceptor reflex is elicited, nerve signals are transmitted simultaneously through skeletal nerves to skeletal muscles of the body.

The rise in cardiac output is an essential ingredient in increasing the arterial pressure during exercise. , Even anticipation of exercise tightens the muscles, thereby compressing the vessels in the muscles and in the abdomen., Translocation of blood from the peripheral vessels into the heart is essential in increasing the cardiac output during heavy exercise., When the skeletal muscles contract during exercise, they compress blood vessels throughout the body., The compression of the vessels from muscles and abdomen translocates blood from the peripheral vessels into the heart and increases cardiac output.

An increase in atrial pressure could increase heart rate as much as 75%., The cerebral ischemia strongly stimulates the vasoconstrictor and cardioaccelerator neurons., The CNS ischemic response operates as an emergency pressure control system that acts powerfully when blood flow to the brain decreases dangerously close to the lethal level. , The ability of the baroreceptors to maintain relatively constant arterial pressure in the upper body is important when a person stands up after having been lying down., The Cushing reaction helps protect vital centers of the brain from loss of nutrition if the cerebrospinal fluid pressure ever rises high enough to compress the cerebral arteries.

The baroreceptors do not completely reset and may contribute to long-term blood pressure regulation, especially by influencing sympathetic nerve activity of the kidneys, Long-term regulation of mean arterial pressure by the baroreceptors requires interaction with additional systems, principally the renal– body fluid–pressure control system. , When the arterial pressure falls to a very low level, the baroreceptors at first transmit no impulses, but after 1 to 2 days, the rate firing returns toward the control level. , After increases in arterial pressure, the baroreceptor reflexes may mediate decreases in renal sympathetic nerve activity. , If the arterial pressure rises and remain to 160 mm Hg, the rate of baroreceptors firing is a very high at the beginning but returned to nearly normal after 1 to 2 days.

The chemoreceptors are in carotid bodies and aortic bodies., Chemoreceptors are always in close contact with arterial blood., The signals transmitted from the chemoreceptors are carried by nerve fibers from Hering’s nerves and the vagus nerves towards the vasomotor center of the brain stem. , Each carotid or aortic body is supplied with an abundant blood flow through a small nutrient artery., The chemoreceptors are chemosensitive cells sensitive to low O2, excess CO2, and excess H+.

The baroreceptors attenuate blood pressure changes during changes in body posture. , Atrial stretch caused by increased blood volume releases atrial natriuretic peptide., The CNS ischemic response is one of the most powerful of all the activators of the sympathetic vasoconstrictor system., The signals transmitted from the chemoreceptors excite the vasomotor center, which responds by raising blood pressure back to normal., Stretch of the atria also causes significant reflex dilation of the afferent arterioles in the kidneys.

Atrial stretch increases fluid loss by the kidneys and reduces an increased blood volume back toward normal., Both the atria and the pulmonary arteries have in their walls stretch receptors called low pressure receptors., The increase in heart rate as result of atrial pressure increase is partly caused by the direct stretch of the sinus node., Because the baroreceptor system opposes either increases or decreases in arterial pressure, it is called a pressure buffer system., The chemoreceptor reflex is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm Hg.

Low-pressure receptors play an important role, in minimizing arterial pressure changes in response to changes in blood volume., Stretch of the atria decreases secretion of antidiuretic hormone., The stretch receptors of the atria that elicit the Bainbridge reflex transmit their afferent signals through the vagus nerves to the medulla of the brain., If blood pressure decrease, the chemoreceptors become stimulated because diminished blood flow means decrease of O2, along with increase of CO2 and H+., The primary purpose of the arterial baroreceptor system is to reduce minute-byminute arterial pressure.

With the arterial baroreceptors denervated, the arterial pressure rises about 40 mm Hg. , The low-pressure receptors in the pulmonary artery and in the atria detect increases in pressure in the low-pressure areas of the circulation caused by increase in volume. , Immediately on standing, the decrease of arterial pressure activates the baroreceptor reflex and the sympathetic response that minimizes pressure decrease in the head and upper body. , The Bainbridge reflex: atrial stretch produces the increase heart rate and strength of heart contraction. , Stretch of the atria decreases secretion of antidiuretic hormone and diminishes the reabsorption of water from the tubules.

The chemoreceptor reflex is initiated by chemoreceptors. , The Bainbridge reflex helps prevent damming of blood in the veins, atria, and pulmonary circulation. , Stretch of the atria decreases afferent arteriolar resistance and increases glomerular filtration of fluid into the kidney tubules., The low-pressure receptors maintain the mean atrial pressure around 80 mm Hg., The arterial baroreceptor system reduces the minute-by-minute variation in arterial pressure.

Vasomotor waves caused by the baroreceptor reflex are dampened when the response of the circulatory system is delayed by a few seconds., The vasomotor waves or Mayer waves are pressure waves with amplitude of 10 to 40 mm Hg and duration of each cycle of 7 to 10 seconds in the unanesthetized human. , The cause of vasomotor waves is “reflex oscillation” of one or more nervous pressure control mechanisms, Any reflex pressure control mechanism can oscillate if the intensity of “feedback” is strong enough and if there is a delay between excitation of the pressure receptor and the subsequent pressure response., The vasomotor waves could be caused by oscillation of the baroreceptor reflex when the response of the circulatory system is delayed by several seconds.

The sensory area of the vasomotor center is involved in baroreceptor reflex activity. , The powerful sympathetic vasoconstrictor in intestines, spleen, and skin is mediated by alfa adrenergic receptors. , The stimulation of the anterior temporal lobe, the amygdala, and the hippocampus can either excite or inhibit the vasomotor center. , When there is a need to decrease heart pumping, the medial portion of the vasomotor center stimulate the dorsal motor nuclei of the vagus nerves., During deep respiration, the blood pressure can rise and fall as much as 20 mm Hg with each respiratory cycle.

In a few tissues epinephrine causes vasodilation because it also has a beta adrenergic receptor stimulatory effect, Emotional fainting begins with disturbing thoughts in the cerebral cortex. , The CNS ischemic response is called the “last-ditch stand” pressure control mechanism, With each cycle of respiration, the arterial pressure usually rises and falls 4 to 6 mm Hg in a wavelike manner, causing respiratory waves in the arterial pressure., Sudden inhibition of nervous cardiovascular stimulation can decrease the arterial pressure to as little as one-half normal within less than a minute.

For short-term survival, fluid intake and output must be precisely balanced. , This long-term control of arterial pressure is closely intertwined with homeostasis of body fluid volume, The sympathetic nervous system plays a major role in short-term arterial blood pressure regulation., Multiple nervous and hormonal controls and local control systems within the kidneys play a major role in the precise and short-term balance of fluid intake and output., The sympathetic nervous system plays a major role in short-term arterial blood pressure regulation by increasing the heart pumping ability.

The rising pressure, causes the kidneys to excrete the excess volume, returning the pressure back toward normal., An increase in arterial pressure in the human of only a few mm Hg can double renal output of salt (pressure natriuresis)., The renal–body fluid system for arterial pressure control acts slowly but powerfully. , An increase in arterial pressure in the human of only a few mm Hg can double renal output of water (pressure diuresis). , The renal–body fluid system for arterial pressure control is a fundamental mechanism for long-term arterial pressure control

In the human being, at an arterial pressure of 50 mm Hg, the urine output is essentially zero., The pressure increasing increases both the urine volume output and the sodium output., The phenomenon of pressure natriuresis is initiated by increase of blood pressure. , In the human being, at an arterial pressure of 100 mm Hg, the urine output is normal. , In the human being, at an arterial pressure of 200 mm Hg, the urine output is higher than normal.

Excessive formation of antinatriuretic hormones such as angiotensin II or aldosterone. , Injury to the kidney due to hypertension., Surgical reduction of kidney mass, Loss of functional nephrons due to kidney injury due to various kidney diseases. , Injury to the kidney due to diabetes.

In persons with impaired kidney function, the reduced steepness of the renal output curve may result in increased sensitivity of arterial pressure to changes in salt intake., When the kidneys are functioning normally, the sensitivity of arterial pressure to acute changes in salt intake is much greater than to chronic changes, In persons with impaired kidney function, the steepness of the renal output curve may be reduced, like the acute curve. , When the kidneys are functioning normally, the acute increase of salt and water intake have much greater effect on arterial pressure, than observed during the chronic changes of intake. , When the kidneys are functioning normally, the chronic changes in arterial pressure, lasting for days or months, have much greater effect on renal output of salt and water than observed during acute changes in pressure.

Decreasing activity of the sympathetic nervous system., Reducing activity of the antinatriuretic systems, Decreasing levels of aldosteron, Hemodynamic effects on the kidney to increase excretion, Decreasing levels of angiotensin II

Long-term high salt intake, lasting for several years, may actually damage the kidneys and eventually makes blood pressure more salt sensitive., Persons with excessive secretion of aldosterone may be salt sensitive. , Persons with excessive secretion of angiotensin II may be salt sensitive. , Persons with kidney injury due to various kidney diseases may be salt sensitive., Persons with kidney injury due to hypertension or diabetes may be salt sensitive.

Reduced activity of antinatriuretic systems, Pressure natriuresis , Decrease activity of the sympathetic nervous system , Pressure diuresis, Decrease angiotensin II and aldosterone

↓ Angiotensin II, ↓ Activity of antinatriuretic systems , ↓ Aldosterone secretion , ↓ Sympathetic nervous system activity , ↑ Glomerular filtration (direct hemodynamic effect)

Many persons are said to be salt insensitive because large variations in salt intake do not change blood pressure more than a few mm Hg. , Decreases in salt and water intake to as low as one-sixth normal typically have little effect on arterial pressure., Persons with kidney injury or excessive secretion of aldosterone are sensitive. , Persons with injury to the kidney due to hypertension or diabetes may have blood pressure to be more sensitive to changes in salt intake, Chronic increases in intakes of salt and water to as high as six times normal are usually associated with only small increases in arterial pressure